Pressing and extrusion under very high hydrostatic pressure is an efficient method of processing metals and other materials. Hydraulic presses and cylinders of ever growing power are introduced in metal working and other industries. High-pressure devices need seas capable to work under a wide range of pressures, from atmospheric pressure to extremely high pressures of 20–40 Kbar and more, at which the internal diameter of the high-pressure cylinder expands by ˜1% and more.
A hydraulic cylinder has a cylinder wall with an open end wherein a plunger enters the cylinder to compress a hydraulic liquid to high pressure levels. A seal assembly is disposed in an annular recess in the cylinder wall near the open end. Usually, a high-pressure seal assembly comprises a high-pressure seal subassembly and a low-pressure seal subassembly. With regard to expressions “high-pressure” and “low-pressure”, it will be understood by those skilled in the art that a single pressure value cannot be specified as a border line between the two. What is meant here and further in the disclosure is, that from zero or atmospheric pressure up to some intermediate pressure range, the sealing of the cylinder is provided solely by elements of the so-called “low-pressure subassembly”. At pressures higher than said intermediate pressure range and up to the maximal design pressure, the sealing of the cylinder is provided by the elements of the so-called “high-pressure subassembly” only. In the intermediate pressure range, both subassemblies cooperate to provide sealing. Some elements of the sealing assembly may belong to both subassemblies.
Existing high-pressure seal assemblies usually include elements made of resilient material which, when interacting with the plunger and other metal parts, are subjected to rapid wear. Sometimes, parts of a seal assembly may require to be changed after each pressing cycle.
U.S. Pat. No. 3,606,356 to Beroset presents a high-pressure seal assembly disposed in a recess of a high-pressure cylinder wall. The low-pressure subassembly of U.S. Pat. No. 3,606,356 includes an annular Teflon ring with rectangular profile fitted on the plunger and placed in the recess, an annular Teflon ring with triangular profile embracing the rectangular ring and fitted to the recess outer wall, and a rubber O-ring inserted like a wedge between the two Teflon rings. The high-pressure subassembly includes a metallic antiextrusion ring with triangular profile spanning the recess radially, fitted on the plunger, and abutting the two Teflon rings from the open-end side of the recess. The seal assembly is closed by a retaining ring (seal holder) engaged in a screw-thread at the open end of the cylinder and biasing the antiextrusion ring against the two Teflon rings, thereby urging the rectangular ring and the O-ring against the bottom of the recess. At low pressures, the cylinder is sealed by the two Teflon rings and the O-ring between them. At high pressures, the rectangular Teflon ring and the O-ring cease to operate but the triangular Teflon ring expands under the action of the pressure together with the cylinder wall, and the sealing is provided by the metallic antiextrusion ring in cooperation with the triangular ring. Thus, the triangular Teflon ring actually belongs to the high-pressure subassembly as well.
U.S. Pat. No. 3,865,387 discloses high-pressure seal assemblies including two metal rings disposed in a recess of the cylinder wall similar to that described above. In one embodiment, a metallic “plunger-sealing” ring with triangular profile is fitted to the plunger and sealingly abuts a seal holder fixed over the open end of the recess. A metallic “cylinder-sealing” ring of generally rectangular profile is fitted inside the recess of the cylinder wall, embracing the first ring, and also sealingly abutting the seal holder. The cylinder-sealing ring has an internal skewed recess or bevel at the recess open-end side, accommodating part of the plunger-sealing ring, the two rings defining a narrow conical slit therebetween. The cylinder-sealing ring has two more annular pocket-like recesses, each accommodating a rubber O-ring. One recess is at the beveled internal side opposite the edge of the plunger-sealing ring, and its O-ring is urged to the plunger-sealing ring, to the cylinder-sealing ring, and to the plunger. The other recess is at the outer periphery of the cylinder-sealing ring and its O-ring is urged to the cylinder wall. Low-pressure sealing is provided by the O-rings only. High-pressure sealing is provided by the two metallic rings with the cooperation of the O-ring that is between them.
A second embodiment of the U.S. Pat. No. 3,865,387 has a similar arrangement but the cylinder-sealing ring is beveled also at the recess bottom side. An annular groove is formed between the two metallic rings, accommodating an enlarged O-ring which is urged to both metallic rings, to the plunger and to the cylinder wall. Low-pressure sealing is provided by the O-ring. High-pressure sealing is provided by the two metallic rings with the cooperation of the O-ring.
There are other high-pressure seal designs, where the operation of the high-pressure subassembly does not involve rapid wear of the low-pressure subassembly, but then the high-pressure subassembly experiences too large elastic deformations leading to rapid wear.
Thus, U.S. Pat. No. 4,602,791 discloses a high-pressure seal assembly, where the high-pressure subassembly comprises a plurality of elastic metallic rings with chevron profile (V-profile). The rings are stacked in a recess of the cylinder wall, embracing the plunger, abutting the plunger and the cylinder wall, with the V-profile open towards the high-pressure side of the cylinder. Under the action of the high pressure, the V-profile opens elastically, following the expansion of the cylinder, and seals the gap around the plunger.
U.S. Pat. No. 4,787,642 discloses an arrangement, wherein an X-profile high-pressure metallic ring is disposed in a recess of the cylinder wall. Two deformable rings with triangular profiles are fitted in the lateral spaces of the X-profile, at the plunger and at the cylinder, respectively, constituting the low-pressure subassembly. Two metallic rings with wedge profiles are inserted in the upper and the lower spaces of the X-profile. The operation of this sealing assembly at high pressure is the same as the previous one, the X-profile effectively working as two opposite V-profiles.